Happy Birthday Yard Sign Letters - Happy Birthday Letters Yard Sign Birthday Lawn Decorations Outdoor for Kids or Adults with Metal Stakes. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. These outdoor Happy Birthday Outdoor Lawn Letters Decorations Yard Signs full color direct printed graphics are vertically fluted for use with a Stakes. Coroplast is typically used for yard, parking, real estate, and election signs. They are waterproof.